Induction furnaces for heating or melting metals operate on the principle of inducing eddy currents in the metal charge to be heated. The eddy currents cause the metal charge to act as its own heat source by the P=I.sup.2 R heating principle. The eddy currents are induced in the metal charge by passing alternating current through a coil disposed near or around the metal charge. In a coreless induction furnace, the metal charge is typically of a generally cylindrical shape and is disposed inside the coil, so that the metal charge itself acts as the core.
Coreless induction furnaces in common use today often include induction coils of copper tubing adapted to allow a liquid coolant to flow therethrough. The copper tubing conducts the alternating current which produces the electromagnetic field inside the furnace to create the eddy currents in the metal charge. The copper tubing also serves to support the furnace lining. The furnace lining is typically a refractory material and forms a cylindrical reservoir for the molten metal. Running water or other liquid coolant flows through the copper tubing of the coil to remove the heat conducted through the refractory material and the heat generated by the coil current.
FIGS. 1a and 1b are partial cross-sectional views through two prior art induction furnaces. Each furnace comprises an induction coil 12 which defines a volume in the furnace which receives the metal charge to be heated. When alternating current is passed through the coil 12, a magnetic field is generated, shown in each figure by the pattern of dotted lines which represent magnetic flux lines. The portion of the magnetic field in the interior of the coil passes through the refractory lining or crucible 26 and through the metal charge to be heated. However, a magnetic field of substantially the same magnitude as the field passing through the metal charge extends outward from the exterior of the coil 12.
In large coreless induction furnaces, the hydrostatic pressure of molten metal is usually so large that the coil 12 alone can not support the molten metal bath. To prevent the furnace from being destroyed by the heat and mass of the melting metal, the coil and refractory lining are usually placed within a steel shell 20. The steel shell 20 provides physical support to the coil 12 and the refractory lining 26. However, there are certain problems associated with steel shells.
Even if the outer shell 20 is not ferromagnetic but merely conductive, the magnetic field on the exterior of the coil will cause eddy currents to flow through the shell 20, thereby heating the shell in the same manner as the metal charge inside the coil. This stray magnetic field may also affect other machinery and apparatus in the vicinity of the furnace, and further represents wasted energy. The stray field lowers the power factor of the furnace and reduces the power input per ampere turn in the coil of the furnace inductor, thus limiting the output of any given furnace.
Certain techniques are known in the prior art to protect the top and bottom of the furnace. One common design feature is extra coil turns on the ends of the coil, extending beyond the axial length of the crucible. The extra turns include cooling means but do not carry any current. These non-current-carrying turns are shown as having a round cross-section at 16 and 18 in FIGS. 1A and 1B, as opposed to the current-carrying turns 14, which are shown with a square cross-section. It can be seen that the lower non-current-carrying turns 18 are disposed below the bottom of the crucible, and the top turns 16 are disposed above the top level of the metal charge. This construction of the coil has proven effective in limiting the temperature around the furnace.
Another common technique for reducing the extent of the stray magnetic field outside the induction coil is the use of magnetic shunts, or yokes. As shown in FIG. 1B, yoke 30 is an elongated member disposed outside and adjacent to the coil. Furnaces usually include a plurality of such yokes. Each yoke 30 is of such a length so that magnetic field lines extending outward from the top and bottom of the coil enter into the yoke 30 and pass from one end of the yoke to the other, instead of straying beyond the yoke to the shell 20. Yokes are traditionally assembled from long pieces of transformer steel; that is, a plurality of laminations of steel with insulating layers therebetween. Because transformer steel has a greater magnetic permeability than air, the stray magnetic field will pass through each of the yokes in preference to the surrounding air, with the effect that fewer magnetic field lines will penetrate the outer shell 20, as seen in FIG. 1B.
The width and number of yokes are typically selected so that all of the yokes together cover about 50% of the coil circumference. A common design parameter for induction furnaces with straight (or "stacked") yokes is: EQU W =1/2.pi..multidot.D/N
where W is the yoke width, D is the coil diameter, and N is the number of yokes.
A prior art coreless induction furnace with transformer steel stacked yokes is shown in detail in FIG. 2. A portion of the coil 12 is shown with two yokes 30. The yokes 30 are usually distributed evenly around the entire coil in a similar fashion. In addition to conducting the electromagnetic field, the yokes also provide physical support to the sides of the furnace.
It has been found in the prior art that even though the stacked yokes may be effective in reducing the extent of the stray magnetic field, they do not effectively prevent excessive heat generation in the coil and shell. Magnetic flux coming out of the coil between the yokes is split in the air and enters the yokes obliquely to the orientation of the laminations, as can be seen by the flux arrows 24 in FIG. 2. Consequently there is a great deal of crossing of flux lines where the magnetic field enters and exits the yokes, i.e. at the top and bottom ends of the yokes. The unevenness of the flux lines within the laminations causes eddy currents, with resulting excessive heat within the yokes themselves. Prior art induction furnaces commonly require liquid cooling means for the yokes themselves, such as the plurality of tubes 32 disposed along each of the yokes 30 in FIG. 2, in addition to the cooling system in the coil. Water or other liquid coolant flows through these tubes 32, cooling the yokes 30. The tubes 32 are held against the yoke 30 by brackets 34. The measures that must be taken to remove the excess heat in the yokes increase the cost of the furnace and also reduce furnace efficiency.
An additional disadvantage of straight or "stacked" yokes is that the yokes do not cover 100% of the surface of the side walls of the furnace toward the ends of the coil, and therefore do not completely protect the shell.
It is an object of the present invention to provide an apparatus for reducing the stray magnetic current associated with a coreless induction furnace that is more effective than prior art yokes, and which may be manufactured relatively inexpensively.